Spotlight summary:
In this paper Büttner et al. demonstrate in a very elegant way a new application for adaptive optics (AO) technology: the correction of wavefront distortions in the light transmitted through a fluctuating gas-liquid interface. As an example they integrated an adaptive optics system into the light delivery path of an interferometric velocity measurement system allowing improved flow detection using an illumination beam passing through a fluctuating water surface. This novel demonstration of AO technology will help to extend its application to other optical metrology methods that currently cannot be used in various complex measurement environments due to measuring beam fluctuations.

Despite the increasing number of successful applications of AO in many disciplines including astronomy, vision science, ophthalmology, microscopy and free-space optical communication, its application in optical metrology remains limited. Fortunately, continued progress in AO technology including its increasing commercialization combined with cost reduction should make it more attractive for an increasing number of applications. This is why the authors’ effort to apply AO to the new field of fluid flow research is so valuable. Adaptive optics is potentially ideally suited for implementation in flow research, as there are potentially many sources of dynamic optical distortions that limit application of optical detection methods in this field. In general most distortions are caused by flows of inhomogeneous media or fluctuations at the interface between two media of different refractive indices. The authors of this paper investigate AO correction of the optical distortion of a fluctuating water surface by statistical evaluation of measurements performed by laser Doppler velocimetry (LDV). The paper provides a detailed description of velocity measurement techniques followed by modeling the optical distortions caused by the fluctuating air-water interface. In the experimental part of the manuscript the authors present results from characterizing the amplitude and temporal characteristics of the fluctuating air-water interface, which allowed them to define requirements for the AO system implemented with the LDV experiment that was then described. The presented results include measurements of interference contrast, validation rate and velocity measurement uncertainty as a function of mean amplitude of surface distortion without and with AO correction. Finally, the authors simulate the measurement process to estimate the individual contributions of different optical distortions (aberration modes) of the water interface. Close agreement between the simulation and experiment makes this report even more valuable as it allows calculation of AO system component parameters required for correcting different ranges of water surface fluctuations.